Internet Engineering Task Force (IETF) J. Snell
Request for Comments: 7240 June 2014
Category: Standards Track
ISSN: 2070-1721
Prefer Header for HTTP
Abstract
This specification defines an HTTP header field that can be used by a
client to request that certain behaviors be employed by a server
while processing a request.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7240.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................2
1.1. Syntax Notation ............................................4
2. The Prefer Request Header Field .................................4
2.1. Examples ...................................................6
3. The Preference-Applied Response Header Field ....................7
4. Preference Definitions ..........................................8
4.1. The "respond-async" Preference .............................8
4.2. The "return=representation" and "return=minimal"
Preferences ................................................9
4.3. The "wait" Preference .....................................11
4.4. The "handling=strict" and "handling=lenient" Processing ...12
5. IANA Considerations ............................................13
5.1. The Registry of Preferences ...............................13
5.2. Initial Registry Contents .................................15
6. Security Considerations ........................................16
7. References .....................................................16
7.1. Normative References ......................................16
7.2. Informative References ....................................16
1. Introduction
Within the course of processing an HTTP request, there are typically
a range of required and optional behaviors that a server or
intermediary can employ. These often manifest in a variety of subtle
and not-so-subtle ways within the response.
For example, when using the HTTP PUT method to modify a resource --
similar to that defined for the Atom Publishing Protocol [RFC5023] --
the server is given the option of returning either a complete
representation of a modified resource or a minimal response that
indicates only the successful completion of the operation. The
selection of which type of response to return to the client generally
has no bearing on the successful processing of the request but could,
for instance, have an impact on what actions the client must take
after receiving the response. That is, returning a representation of
the modified resource within the response can allow the client to
avoid sending an additional subsequent GET request.
Similarly, servers that process requests are often faced with
decisions about how to process requests that may be technically
invalid or incorrect but are still understandable. It might be the
case that the server is able to overlook the technical errors in the
request but still successfully process the request. Depending on the
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specific requirements of the application and the nature of the
request being made, the client might or might not consider such
lenient processing of its request to be appropriate.
While the decision of exactly which behaviors to apply in these cases
lies with the server processing the request, the server might wish to
defer to the client to specify which optional behavior is preferred.
Currently, HTTP offers no explicitly defined means of expressing the
client's preferences regarding the optional aspects of handling of a
given request. While HTTP does provide the Expect header -- which
can be used to identify mandatory expectations for the processing of
a request -- use of the field to communicate optional preferences is
problematic:
1. The semantics of the Expect header field are such that
intermediaries and servers are required to reject any request
that states unrecognized or unsupported expectations.
2. While the Expect header field is end to end, the HTTP
specification requires that the header be processed hop by hop.
That is, every interceding intermediary that handles a request
between the client and the origin server is required to process
an expectation and determine whether it is capable of
appropriately handling it.
The must-understand semantics of the Expect header make it a poor
choice for the expression of optional preferences.
Another option available to clients is to utilize Request URI
query-string parameters to express preferences. However, any
mechanism that alters the URI can have undesirable effects, such as
when caches record the altered URI.
As an alternative, this specification defines a new HTTP request
header field that can be used by clients to request that optional
behaviors be applied by a server during the processing the request.
Additionally, a handful of initial preference tokens for use with the
new header are defined.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
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1.1. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] and includes, by reference, the "token",
"word", "OWS", and "BWS" rules and the #rule extension as defined
within Sections 3.2.1 and 3.2.4 of [RFC7230]; as well as the
"delta-seconds" rule defined in Section 8.1.3 of [RFC7231].
2. The Prefer Request Header Field
The Prefer request header field is used to indicate that particular
server behaviors are preferred by the client but are not required for
successful completion of the request. Prefer is similar in nature to
the Expect header field defined by Section 6.1.2 of [RFC7231] with
the exception that servers are allowed to ignore stated preferences.
ABNF:
Prefer = "Prefer" ":" 1#preference
preference = token [ BWS "=" BWS word ]
*( OWS ";" [ OWS parameter ] )
parameter = token [ BWS "=" BWS word ]
This header field is defined with an extensible syntax to allow for
future values included in the Registry of Preferences (Section 5.1).
A server that does not recognize or is unable to comply with
particular preference tokens in the Prefer header field of a request
MUST ignore those tokens and continue processing instead of signaling
an error.
Empty or zero-length values on both the preference token and within
parameters are equivalent to no value being specified at all. The
following, then, are equivalent examples of a "foo" preference with a
single "bar" parameter.
Prefer: foo; bar
Prefer: foo; bar=""
Prefer: foo=""; bar
An optional set of parameters can be specified for any preference
token. The meaning and application of such parameters is dependent
on the definition of each preference token and the server's
implementation thereof. There is no significance given to the
ordering of parameters on any given preference.
For both preference token names and parameter names, comparison is
case insensitive while values are case sensitive regardless of
whether token or quoted-string values are used.
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The Prefer header field is end to end and MUST be forwarded by a
proxy if the request is forwarded unless Prefer is explicitly
identified as being hop by hop using the Connection header field
defined by [RFC7230], Section 6.1.
In various situations, a proxy might determine that it is capable of
honoring a preference independently of the server to which the
request has been directed. For instance, an intervening proxy might
be capable of providing asynchronous handling of a request using 202
(Accepted) responses independently of the origin server. Such
proxies can choose to honor the "respond-async" preference on their
own regardless of whether or not the origin is capable or willing to
do so.
Individual preference tokens MAY define their own requirements and
restrictions as to whether and how intermediaries can apply the
preference to a request independently of the origin server.
A client MAY use multiple instances of the Prefer header field in a
single message, or it MAY use a single Prefer header field with
multiple comma-separated preference tokens. If multiple Prefer
header fields are used, it is equivalent to a single Prefer header
field with the comma-separated concatenation of all of the tokens.
For example, the following are equivalent:
Multiple Prefer header fields defining three distinct preference
tokens:
POST /foo HTTP/1.1
Host: example.org
Prefer: respond-async, wait=100
Prefer: handling=lenient
Date: Tue, 20 Dec 2011 12:34:56 GMT
A single Prefer header field defining the same three preference
tokens:
POST /foo HTTP/1.1
Host: example.org
Prefer: handling=lenient, wait=100, respond-async
Date: Tue, 20 Dec 2011 12:34:56 GMT
To avoid any possible ambiguity, individual preference tokens SHOULD
NOT appear multiple times within a single request. If any preference
is specified more than once, only the first instance is to be
considered. All subsequent occurrences SHOULD be ignored without
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signaling an error or otherwise altering the processing of the
request. This is the only case in which the ordering of preferences
within a request is considered to be significant.
Due to the inherent complexities involved with properly implementing
server-driven content negotiation, effective caching, and the
application of optional preferences, implementers are urged to
exercise caution when using preferences in a way that impacts the
caching of a response and SHOULD NOT use the Prefer header mechanism
for content negotiation. If a server supports the optional
application of a preference that might result in a variance to a
cache's handling of a response entity, a Vary header field MUST be
included in the response listing the Prefer header field regardless
of whether the client actually used Prefer in the request.
Alternatively, the server MAY include a Vary header with the special
value "*" as defined by [RFC7231], Section 8.2.1. Note, however,
that use of the "Vary: *" header will make it impossible for a proxy
to cache the response.
Note that while Preference tokens are similar in structure to HTTP
Expect tokens, the Prefer and Expect header fields serve very
distinct purposes and preferences cannot be used as expectations.
2.1. Examples
The following examples illustrate the use of various preferences
defined by this specification, as well as undefined extensions for
strictly illustrative purposes:
1. Return a 202 (Accepted) response for asynchronous processing if
the request cannot be processed within 10 seconds. An undefined
"priority" preference is also specified:
POST /some-resource HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: respond-async, wait=10
Prefer: priority=5
{...}
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2. Use lenient processing:
POST /some-resource HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: Lenient
{...}
3. Use of an optional, undefined parameter on the return=minimal
preference:
POST /some-resource HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return=minimal; foo="some parameter"
{...}
3. The Preference-Applied Response Header Field
The Preference-Applied response header MAY be included within a
response message as an indication as to which Prefer tokens were
honored by the server and applied to the processing of a request.
ABNF:
Preference-Applied = "Preference-Applied" ":" 1#applied-pref
applied-pref = token [ BWS "=" BWS word ]
The syntax of the Preference-Applied header differs from that of the
Prefer header in that parameters are not included.
Use of the Preference-Applied header is only necessary when it is not
readily and obviously apparent that a server applied a given
preference and such ambiguity might have an impact on the client's
handling of the response. For instance, when using either the
"return=representation" or "return=minimal" preferences, a client
application might not be capable of reliably determining if the
preference was (or was not) applied simply by examining the payload
of the response. In such a case, the Preference-Applied header field
can be used.
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Request:
PATCH /my-document HTTP/1.1
Host: example.org
Content-Type: application/example-patch
Prefer: return=representation
[{"op": "add", "path": "/a", "value": 1}]
Response:
HTTP/1.1 200 OK
Content-Type: application/json
Preference-Applied: return=representation
Content-Location: /my-document
{"a": 1}
4. Preference Definitions
The following subsections define an initial set of preferences.
Additional preferences can be registered for convenience and/or to
promote reuse by other applications. This specification establishes
an IANA registry of preferences (see Section 5.1).
4.1. The "respond-async" Preference
The "respond-async" preference indicates that the client prefers the
server to respond asynchronously to a response. For instance, in the
case when the length of time it takes to generate a response will
exceed some arbitrary threshold established by the server, the server
can honor the "respond-async" preference by returning a 202
(Accepted) response.
ABNF:
respond-async = "respond-async"
The key motivation for the "respond-async" preference is to
facilitate the operation of asynchronous request handling by allowing
the client to indicate to a server its capability and preference for
handling asynchronous responses.
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An example request specifying the "respond-async" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: respond-async
{Data}
An example asynchronous response using 202 (Accepted):
HTTP/1.1 202 Accepted
Location: http://example.org/collection/123
While the 202 (Accepted) response status is defined by [RFC7231],
little guidance is given on how and when to use the response code and
the process for determining the subsequent final result of the
operation is left entirely undefined. Therefore, whether and how any
given server supports asynchronous responses is an implementation-
specific detail that is considered to be out of the scope of this
specification.
4.2. The "return=representation" and "return=minimal" Preferences
The "return=representation" preference indicates that the client
prefers that the server include an entity representing the current
state of the resource in the response to a successful request.
The "return=minimal" preference, on the other hand, indicates that
the client wishes the server to return only a minimal response to a
successful request. Typically, such responses would utilize the 204
(No Content) status, but other codes MAY be used as appropriate, such
as a 200 (OK) status with a zero-length response entity. The
determination of what constitutes an appropriate minimal response is
solely at the discretion of the server.
ABNF:
return = "return" BWS "=" BWS ("representation" / "minimal")
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When honoring the "return=representation" preference, the returned
representation might not be a representation of the effective request
URI when the request is affecting another resource. In such cases,
the Content-Location header can be used to identify the URI of the
returned representation.
The "return=representation" preference is intended to provide a means
of optimizing communication between the client and server by
eliminating the need for a subsequent GET request to retrieve the
current representation of the resource following a modification.
After successfully processing a modification request such as a POST
or PUT, a server can choose to return either an entity describing the
status of the operation or a representation of the modified resource
itself. While the selection of which type of entity to return, if
any at all, is solely at the discretion of the server, the
"return=representation" preference -- along with the "return=minimal"
preference defined below -- allow the server to take the client's
preferences into consideration while constructing the response.
An example request specifying the "return=representation" preference:
PATCH /item/123 HTTP/1.1
Host: example.org
Content-Type: application/example-patch
Prefer: return=representation
1c1
< ABCDEFGHIJKLMNOPQRSTUVWXYZ
---
> BCDFGHJKLMNPQRSTVWXYZ
An example response containing the resource representation:
HTTP/1.1 200 OK
Content-Location: http://example.org/item/123
Content-Type: text/plain
ETag: "d3b07384d113edec49eaa6238ad5ff00"
BCDFGHJKLMNPQRSTVWXYZ
In contrast, the "return=minimal" preference can reduce the amount of
data the server is required to return to the client following a
request. This can be particularly useful, for instance, when
communicating with limited-bandwidth mobile devices or when the
client simply does not require any further information about the
result of a request beyond knowing if it was successfully processed.
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An example request specifying the "return=minimal" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return=minimal
{Data}
An example minimal response:
HTTP/1.1 201 Created
Location: http://example.org/collection/123
The "return=minimal" and "return=representation" preferences are
mutually exclusive directives. It is anticipated that there will
never be a situation where it will make sense for a single request to
include both preferences. Any such requests will likely be the
result of a coding error within the client. As such, a request
containing both preferences can be treated as though neither were
specified.
4.3. The "wait" Preference
The "wait" preference can be used to establish an upper bound on the
length of time, in seconds, the client expects it will take the
server to process the request once it has been received. In the case
that generating a response will take longer than the time specified,
the server, or proxy, can choose to utilize an asynchronous
processing model by returning -- for example -- a 202 (Accepted)
response.
ABNF:
wait = "wait" BWS "=" BWS delta-seconds
It is important to consider that HTTP messages spend some time
traversing the network and being processed by intermediaries. This
increases the length of time that a client will wait for a response
in addition to the time the server takes to process the request. A
client that has strict timing requirements can estimate these factors
and adjust the wait value accordingly.
As with other preferences, the "wait" preference could be ignored.
Clients can abandon requests that take longer than they are prepared
to wait.
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For example, a server receiving the following request might choose to
respond asynchronously if processing the request will take longer
than 10 seconds:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: respond-async, wait=10
{Data}
4.4. The "handling=strict" and "handling=lenient" Processing
Preferences
The "handling=strict" and "handling=lenient" preferences indicate, at
the server's discretion, how the client wishes the server to handle
potential error conditions that can arise in the processing of a
request. For instance, if the payload of a request contains various
minor syntactical or semantic errors, but the server is still capable
of comprehending and successfully processing the request, a decision
must be made to either reject the request with an appropriate "4xx"
error response or go ahead with processing. The "handling=strict"
preference can be used to indicate that, while any particular error
may be recoverable, the client would prefer that the server reject
the request. The "handling=lenient" preference, on the other hand,
indicates that the client wishes the server to attempt to process the
request.
ABNF:
handling = "handling" BWS "=" BWS ("strict" / "lenient")
An example request specifying the "strict" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: handling=strict
The "handling=strict" and "handling=lenient" preferences are mutually
exclusive directives. It is anticipated that there will never be a
situation where it will make sense for a single request to include
both preferences. Any such requests will likely be the result of a
coding error within the client. As such, a request containing both
preferences can be treated as though neither were specified.
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5. IANA Considerations
The 'Prefer' and 'Preference-Applied' header fields have been added
to the "Permanent Message Header Field Names" registry defined in
[RFC3864] (http://www.iana.org/assignments/message-headers).
Header field name: Prefer
Applicable Protocol: HTTP
Status: Standard
Author: James M Snell
Change controller: IETF
Specification document: this specification, Section 2
Header field name: Preference-Applied
Applicable Protocol: HTTP
Status: Standard
Author: James M Snell
Change controller: IETF
Specification document: this specification, Section 3
5.1. The Registry of Preferences
IANA has created a new registry, "HTTP Preferences", under the
"Hypertext Transfer Protocol (HTTP) Parameters" registry. New
registrations will use the Specification Required policy [RFC5226].
The requirements for registered preferences are described in
Section 4.
Registration requests consist of the completed registration template
below, typically published in the required specification. However,
to allow for the allocation of values prior to publication, the
Designated Expert can approve registration based on a separately
submitted template once they are satisfied that a specification will
be published. Preferences can be registered by third parties if the
Designated Expert determines that an unregistered preference is
widely deployed and not likely to be registered in a timely manner.
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The registration template is:
o Preference: (A value for the Prefer request header field that
conforms to the syntax rule given in Section 2)
o Value: (An enumeration or description of possible values for the
preference token).
o Optional Parameters: (An enumeration of optional parameters, and
their values, associated with the preference token).
o Description:
o Reference:
o Notes: [optional]
The "Value" and "Optional Parameters" fields MAY be omitted from the
registration template if the specific preference token definition
does not define either.
Registration requests should be sent to the
mailing list, marked clearly in the subject line (e.g., "NEW
PREFERENCE - example" to register an "example" preference). Within
at most 14 days of the request, the Designated Expert(s) will either
approve or deny the registration request, communicating this decision
to the review list and IANA. Denials should include an explanation
and, if applicable, suggestions as to how to make the request
successful.
The Expert Reviewer shall ensure:
o That the requested preference name conforms to the token rule in
Section 2 and that it is not identical to any other registered
preference name;
o That any associated value, parameter names, and values conform to
the relevant ABNF grammar specifications in Section 2;
o That the name is appropriate to the specificity of the preference;
i.e., if the semantics are highly specific to a particular
application, the name should reflect that, so that more general
names remain available for less specific uses.
o That requested preferences do not constrain servers, clients, or
any intermediaries to any behavior required for successful
processing; and
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o That the specification document defining the preference includes a
proper and complete discussion of any security considerations
relevant to the use of the preference.
5.2. Initial Registry Contents
The "HTTP Preferences" registry's initial contents are:
o Preference: respond-async
o Description: Indicates that the client prefers that the server
respond asynchronously to a request.
o Reference: [this specification], Section 4.1
o Preference: return
o Value: One of either "minimal" or "representation"
o Description: When the value is "minimal", it indicates that the
client prefers that the server return a minimal response to a
request. When the value is "representation", it indicates that
the client prefers that the server include a representation of the
current state of the resource in response to a request.
o Reference: [this specification], Section 4.2
o Preference: wait
o Description: Indicates an upper bound to the length of time the
client expects it will take for the server to process the request
once it has been received.
o Reference: [this specification], Section 4.3
o Preference: handling
o Value: One of either "strict" or "lenient"
o Description: When value is "strict", it indicates that the client
wishes the server to apply strict validation and error handling to
the processing of a request. When the value is "lenient", it
indicates that the client wishes the server to apply lenient
validation and error handling to the processing of the request.
o Reference: [this specification], Section 4.4
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6. Security Considerations
Specific preferences requested by a client can introduce security
considerations and concerns beyond those discussed within HTTP/1.1
[RFC7230] and its associated specification documents (see [RFC7230]
for the list of associated works). Implementers need to refer to the
specifications and descriptions of each preference to determine the
security considerations relevant to each.
A server could incur greater costs in attempting to comply with a
particular preference (for instance, the cost of providing a
representation in a response that would not ordinarily contain one;
or the commitment of resources necessary to track state for an
asynchronous response). Unconditional compliance from a server could
allow the use of preferences for denial of service. A server can
ignore an expressed preference to avoid expending resources that it
does not wish to commit.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
June 2014.
7.2. Informative References
[RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
Protocol", RFC 5023, October 2007.
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Author's Address
James M Snell
EMail: jasnell@gmail.com
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